1. Field of the Invention
The present invention relates to a solid-state imaging device of an amplification type, in which a signal generated in each picture element by photoelectric conversion is amplified in each picture element.
2. Related Background Art
An amplification type solid-state imaging device using a MOS transistor has a circuit configuration shown in FIG. 16.
One picture element (PXL) comprises a photodiode (photoelectric conversion element) 101, a transfer switch 102, a reset switch (transistor for reset) 103, a selection switch (transistor for selection) 104 and an amplification transistor (transistor for amplification) 105.
Each picture element (PXL) is connected to a vertical output line 106.
Such picture elements (PXL) are two-dimensionally arranged on one and same flat surface and constitute a picture element array area (PXA).
A constant current source 107 connected to the vertical output line 106 of each column is connected so as to make the MOS transistor 105 operated as a source follower.
Also, a readout circuit is connected to each vertical output line 106 of each column.
The readout circuit, as described later, is separated into two systems: a readout system for reading out a noise signal including the noise generated therein (hereinafter referred to as “N signal”) and a readout system for reading out a signal obtained by adding an optical signal and N signal (hereinafter referred to as “(S+N) signal”). Each readout system has either an N signal transfer switch 110 and an N signal storage capacity 112 or a (S+N) signal transfer switch 111 and a (S+N) signal storage capacity 113.
Moreover, the two readout systems are connected respectively to input lines 116 and 117 of a differential amplifier 115 via a switch 114 for horizontal scanning.
Next, the outline of the operation of this device will be described with reference to a drive timing chart shown in FIG. 17.
A high level reset control pulse φrst which turns on the reset switch 103 of the picture element in a selected horizontal line is given.
Then, a high level selection control pulse φsel which turns off the reset switch 103 and turns on the selection transistor 104 is given.
In this manner, a high level N signal transfer pulse φtn which turns on the N signal transfer switch 110 is given in order to store the N signal outputted to the vertical output line 106 in the N signal storage capacity 112.
When a high level transfer control pulse φtx which turns on the transfer switch 102 of the picture element of the selected row is given and the optical signal stored in the photodiode 101 is inputted to a gate of the amplification transistor 105, an output signal corresponding to the optical signal is obtained from a source of the amplification transistor. In this state, even if the transfer control pulse φtx is reduced to a low level and the transfer switch 102 is turned off, the gate potential of the amplification transistor is kept at a value based on the optical signal.
In this state, a high level (S+N) signal transfer pulse φts which turns on the (S+N) signal transfer switch 111 is given in order to store the (S+N) signal outputted to the vertical output line 106 in the (S+N) signal storage capacity 113.
Then, by turning off the selection switch 104, the readout of the signals from each picture element of the selected row is completed.
The N signal held in the N signal storage capacity 112 contains a KTC noise at the time of a reset and a fixed pattern noise due to variations in a threshold value of the MOS transistor.
The (S+N) signal held by the (S+N) signal storage capacity is a signal due to photoelectric charge added with the reset signal containing the above-described noise.
In this manner, each switch is controlled according to the timing of FIG. 17, and after a signal of the picture element on one horizontal line is held by the above-described two storage capacities of each column, by turning on the switch 114 by a horizontal scanning circuit, the N signal and the (S+N) signal are read out to each input line of the differential amplifier 115. At this time, from the differential amplifier 115, the above-described noises contained in each signal are removed and the signal corresponding to the signal alone due to photoelectric charge is outputted as a sensor output.
In short, by sequentially turning on and off the switch 114 of each column by a horizontal scanning circuit, the signals of the picture elements on one horizontal line are outputted from the differential amplifier. In order to perform this for each horizontal line, every time a picture element row is selected by a vertical scanning circuit, each switch is controlled according to the timing of FIG. 17 and the row is sequentially scanned by the horizontal scanning circuit and this procedure is repeated.
In this manner, the signals of all the picture elements can be outputted from the differential amplifier 115.
In the conventional solid-state imaging device, as the number of picture elements becomes large, a large shading appears in the output signal.